A one-parameter model was developed to explain the effects of birth order and family size on intelligence. Intellectual development within the family context is conceived of as depending on the cumulative effects of the intellectual environment, which for the purposes of the model, consists primarily of the siblings' and parents' intelligence. Mutual influences, through time, on the intellectual development of the siblings are described by the growth parameter alpha. The model predicts both positive as well as negative effects of birth order, a necessarily negative effect of family size, and a handicap for the last born and the only child. The model explains accurately several features of a large birth order study carried out in the Netherlands on nearly 400,000 nineteen-year olds. The major implication of the model is that age gaps between siblings make a substantial contribution to the intellectual environment, which in turn determines the intellectual development of the siblings. The data which the model explains are aggregate data for which age separation between siblings was not available. A thorough test of the model requires age gap information, and as one part of the project such information will be secured. As a second part, information about parents' intelligence will also be taken, as well as information about demographic background, in an effort to establish how much variance in intelligence, relative to other factors, is accounted for just by family size and birth order--variables which represent the contribution of the individual's immediate intellectual environment to his intellectual growth. Changes in adult intelligence will also be examined. Intelligence of adopted children who come into childless homes will be compared with intelligence of adopted children into larger families. BIBLIOGRAPHIC REFERENCES: R. B. Zajonc and Gregory B. Markus. Birth order and intellectual development. Psychological Review, 1975, 82, 74-88. R. B. Zajonc. Family configuration and intelligence. Science, 1976, 192, 227-236.